The present invention relates to a permanent magnet type step motor, and more specifically, to a permanent magnet type step motor which can considerably reduce the leakage of magnetic flux to the exterior thereof.
A permanent magnet type step motor, which includes a permanent magnet in a magnetic path and in which a rotational shaft is rotated and driven stepwise by synthesization with a magnetomotive force of a stator winding, has been heretofore extensively known for use as a step motor, for example, for driving a magnetic head of a magnetic disc device. However, where such a permanent magnet type step motor is used for the magnetic disc device, if the leakage magnetic flux from the motor is great, the magnetic disc is adversely affected. In view of the foregoing, it has been to arrange brackets on either side of a stator core to constitute an outer case formed of a magnetic material.
However, according to the above-described construction, since the whole bracket including a bearing housing is formed of a magnetic material, the construction suffers the disadvantage that magnetic flux from the permanent magnet tends to concentrate on the brackets, as a consequence of which the magnetic flux density of the bracket portion increases and the leakage magnetic flux from the bracket increases accordingly. Further disadvantages of such a construction include that the brackets tend to be heavy thereby making it difficult to form a step motor of smaller size and of light-weight configuration.